1. Field of the Invention
The present invention relates to the field of computer networking, specifically to the field of hub-based communications networks. More specifically, the present invention relates to methods and apparatus providing for a multiport router in which an asynchronous transfer mode (ATM) switch is utilized as a backplane such as may be implemented in a local area network (LAN) hub.
2. Description of the Related Art
The present invention relates to the field of ATM and similar networking system. Such systems are characterized by use of high-speed switches which act to switch message cells of a fixed size and format through the network. Below is provided a general description of ATM networks. The present invention further relates to the field of networked communications systems employing centralized concentrators or hubs which allow interconnection of devices in what is sometimes termed a star configuration. Further, the present invention relates to use of a router for routing information packets between devices interconnected by centralized concentrator.
ATM Networks
The preferred embodiment of the present invention is implemented utilizing an asynchronous transfer mode (ATM) switch as a router in a centralized concentrator. Such ATM switches are well-known in the art and, in fact, are described in various references. One such reference is Handel, R. and Huber, M. N., Integrated Broadband Networks, an Introduction to ATM-based Networks, published by Addison-Wesley Publishing Company, 1991 and hereinafter referred to as the Handel et al. reference. Another such reference is de Prycher, M., Asynchronous Transfer Mode solution for broadband ISDN, published by Ellis Horwood Limited, West Sussex, England, 1991.
In ATM switches information is communicated in fixed-size cells which comprise a well-defined and size-limited header area and user information area. ATM switches may utilize a variety of switching architectures including, for example, a backplane bus architecture (such as has been described by Hughes LAN Systems, Inc. and announced under the trademark Enterprise Hub.TM.), a matrix switching architecture (such as has been described with reference to both Handel et al. and de Prycher) as preferred by the present invention or other architectures, as will be mentioned briefly below. It is noted that the preferred embodiment of the present invention utilizes a matrix switching architecture for its ATM switch; however, it is thought that many of the teachings of the present invention have equal application the various other architectures mentioned herein.
The matrix switching architecture provides for switching of cells through a switch fabric which is designed to act upon information in the header area in order to provide for routing of cells in the network. The switch fabric is normally implemented in hardware, for example using large-scale integrated circuits, in order to provide for high-speed switching of cells in the network.
Standards have been adopted for ATM networks, for example, by the International Telegraph and Telephone Consultative Committee (CCITT). The CCITT standards require a header area comprising a fixed set of fields and being of a fixed size and a payload area, also referred to as a user information area, and also of a fixed size but allowing user-defined information fields. The CCITT standards define the header to be of a very limited size to keep at a minimum overhead associated with each cell
ATM Cell Format
In an ATM network, all information to be transferred is packed into fixed-size slots which are commonly referred to as cells. Of course, such fixed-size slots may be referred to with other terminology, for example, packets. In one standard ATM format (CCITT Recommendation I.150, "B-ISDN ATM Functional Characteristics", Geneva, 1991), the format is generally shown in FIG. 1(a) and includes a 5-byte (also called octet) header field 101 and a 48-byte information field 102. The information field is defined by the standard to be available to the user and the header field is defined by the standard to carry information pertaining to ATM functionality, in particular, information for identification of the cells by means of a label. See, Handel et al., at pages 14-17.
The standardized format for the header field 101 is better shown in FIG. 1(b) and 1(c) and is described in greater detail with reference to Handel et al., at pages 84-91. The header field 101 will be discussed in greater detail below; however, it is worthwhile mentioning here that the header field 101 comprises two fields: (1) a virtual channel identifier (VCI) and (2) a virtual path identifier (VPI). The VPI field is defined as an eight-bit field in one format (see FIG. 1(b)) and as a twelve-bit field in another format (see FIG. 1(c)) and is defined to be used for routing of the cell. The VCI field is also used for routing in the defined format and is defined as a sixteen-bit field.
The de Prycher reference further describes the format of the ATM cell, for example at pages 55-124 and, especially at pages 106-108.
ATM Switching
Two primary tasks are generally accomplished by an ATM switch: (1) translation of VPI/VCI information and (2) transport of cells from the input port to the correct output port. The functions of an ATM switch are more fully described in Handel et al. at pages 113-136.
A switch is typically constructed of a plurality of switching elements which act together to transport a cell from the input of the switch to the correct output. Various types of switching elements are well-known such as the already-mentioned matrix switching elements and bus-type switching elements. In addition, an ATM switch may utilize central memory switching elements, and ring-type switching elements. Each of these are discussed in greater detail in the Handel et al. reference and each carries out the above-mentioned two primary tasks.
Translation of the VPI/VCI information is important because in a standard ATM network the contents of these fields only has local meaning (i.e., the same data would be interpreted differently by each switch). Thus, the VPI/VCI information is translated by each switch and changed prior to the cell being output from the switch. This translation is accomplished through use of translation tables which are loaded into the switch fabric, generally under control of a switch controller.
Importantly, it is the switch fabric, as controlled by the translation tables, which provides for making routing decisions within the switch. The translation tables may updated from time-to-time in order to provide for new virtual paths/virtual circuits or to remove existing ones (this process is sometimes referred to as call set-up and tear-down). Thus, a VPI/VCI is supplied in the cell header at the input of the switch and the VPI/VCI is translated by the switch fabric and the cell is routed to the appropriate output port. However, generally, the device generating the cell has no knowledge of the specific output port on which switch will send the cell. Rather, this routing decision is made by the switch based on the then current translation tables.
As will be seen, the present invention provides for an ATM switch having preconfigured VPI/VCIs and allows for selection of an appropriate output path by the device generating the cell (i.e., the device generating the cell provides for the routing decision rather than the ATM switch.)
Network Concentrators
Network concentrators or hubs are well-known in the art. Two well-known examples of network concentrators are the SynOptics' LattisNet System 2000.TM. and LattisNet System 3000.TM. concentrators. The concentrators are better described in "LattisNet.RTM. Product Overview, A comprehensive Description of the Lattisnet Product Family" (the "LattisNet Product Brochure"), available from SynOptics Communications, Inc. of Santa Clara, Calif., the assignee of the present invention.
Network concentrators typically comprise a number modules, each module having a plurality of ports to which local area network segments may be connected. A local area network segment may support one or more devices such as data terminals, computers, file servers, printers, etc. The various modules are interconnected through a backplane bus or the like in the concentrator module. Thus, a device attached to module 1 may communicate an information packet to the devices attached to the other modules by providing communicating the packet onto the local area network segment connected with module 1 and, module 1, when it receives the packet, communicating the packet onto the backplane bus. The other modules may then receive the packet from the backplane bus and communicate the packet to the devices coupled with network segments connected to the other modules. The various modules may support, for example, Ethernet (or other CSMA/CD protocol), Token Ring and/or FDDI networks.
Objects of the Invention
It is an object of the present invention to utilize an ATM switch as a routing backplane in a network concentrator to provide for relatively fast routing of information packets between LAN segments coup led to the network concentrator.
This and other objects of the present invention will be understood with reference to the detailed description of the preferred embodiment and the accompanying drawings.